The present invention relates in general to fiber optic technology and more particularly to a method and apparatus for modulating light.
Optical communication involves the need of modulating the optical signals.
This modulation may be performed by direct modulation of the light source, e.g. by direct modulation of a laser diode with data signals, but it is preferred in several connections to perform modulation on the light itself. This type of modulation is called optical modulation.
A problem involved by the direct optical modulation is that the characteristics of the modulator change over time as a function of temperature, changed properties of the material, etc. This change specifically means that the optical output as a function of the electrical driver signals is not constant, which results in undesired distortion of the optical signal.
U.S. Pat. No. 5,170,274 describes an optical transmitter of the Mach-Zehnder type, in which feedback from the modulator is obtained by means of a low-frequency pilot tone superimposed on the electrical driver signal of the modulator, thereby providing some compensation for the drift of the operating point of the modulator. However, this transmitter has the drawback that the extinction ratio is gradually impaired because of amplified drift of the modulated signal, or more particularly a drift with respect to the amplitude of the transmission characteristic of the Mach Zehnder modulator.
From the foregoing, it may be appreciated by those skilled in the art that a need has arisen for an improved technique for modulating light. In accordance with the present invention, a system and method for modulating light are provided which substantially eliminate or greatly reduce disadvantages and problems associated with conventional modulation techniques.
The photoelectrical converter circuit is connected to and generates a feedback signal for a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted, in response to the pilot tone signal fed from the pilot tone generator and the feedback signal fed from the photoelectrical converter circuit, to generate driver control signals for the driver circuit to regulate the driver signal fed to the Mach-Zehnder modulator, it is possible to achieve a very well-defined regulation of a modulator circuit. Thus, the invention enables regulation, optimum in some cases, of the fed driver signal, i.e. the modulation amplitude, which is fed to the Mach-Zehnder modulator. The regulation may thus be performed automatically in connection with temperature errors or fluctuations.
This may be a particular advantage in connection with e.g. so-called bias-free Mach-Zehnder modulators, where bias corrections should be avoided, since such bias corrections will cause drift in the overall circuitry.
In case of drift or e.g. temperature fluctuations, it is thus possible according to the invention to control the amplitude of the modulation signal.
It should be noted that it is not decisive according to the invention how the modulation signals, formed by the driver signal and the pilot tone, are fed to the Mach-Zehnder modulator. The pilot tone and the driver signal may e.g. may be mixed electrically before the signal is fed to the electrodes of the modulator, and the driver signal and the pilot tone may be fed to separate electrodes or sets of electrodes in the Mach-Zehnder modulator, if desired.
The pilot tone signal is fed to the logic xe2x80x9c0xe2x80x9d and/or logic xe2x80x9c1xe2x80x9d of the modulation signal, and the feedback circuit provides driver control signals in response to the pilot tone component of the feedback signal corresponding to the pilot tone signal, a particularly advantageous embodiment of the invention is obtained, as the regulation of the modulation voltage is xe2x80x9ctunedxe2x80x9d to either the ideal logic xe2x80x9c0xe2x80x9d or the ideal logic xe2x80x9c1xe2x80x9d. The amplitude regulation of the modulation voltage may thus be adjusted to a desired asymmetric proportion without bias regulation, which is extremely advantageous in several applications, since regulation of the bias/DC operating point per se results in electrode drift.
Regulation of the amplitude thus makes it possible to currently allow for e.g. temperature-caused changes in the transmission characteristic which particularly concern the actual amplitude of the transmission characteristic.
Thus, the regulation according to the invention may thus allow for the undesirability of applying a DC voltage to the Mach-Zehnder modulator, as this DC voltage gives rise to drift in the transmission characteristic.
The driver signal, in response to the pilot tone signal fed to the modulation signal, is regulated to the logic xe2x80x9c0xe2x80x9d or logic xe2x80x9c1xe2x80x9d of the modulation signal in according with either
xcex94Vampxcx9cFLxc2x7k1 
or
xcex94ampxcx9cFHxc2x7k2 
where xcex94Vamp is the change in the driver signal provided by the regulation, k1 and k2 are a suitably selected constant for the algorithm, F1 and FH assume the value +1 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, is in in-phase with the corresponding pilot tone generated by the pilot tone generator, and FL and FH assume the value xe2x88x921 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, is in antiphase with the corresponding pilot tone generated by the pilot tone generator, a very efficient regulation algorithm is achieved according to the invention, as the feedback circuit hooks on to either the logic xe2x80x9c0xe2x80x9d or the logic xe2x80x9c1xe2x80x9d and the corresponding pilot tone. It should be stressed that actually it is immaterial whether pilot tones are generated at both logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, provided it is a pilot tone reference corresponding to the one of the above-mentioned algorithms which has not been selected.
The amplitude regulation of the modulation signal thus gives an improved extinction ratio, i.e. the ratio of logic xe2x80x9c1xe2x80x9d (p1) to logic xe2x80x9c0xe2x80x9d (p0) is increased, and the resulting signal/noise ratio in a complete application may therefore be increased. It should be noted in this connection that this improved extinction ratio is achieved, even though the amplitude regulation provides an asymmetric modulation of the transmission characteristic, as one extreme of the amplitude is adjusted to the ideal logic xe2x80x9c1xe2x80x9d or logic xe2x80x9c0xe2x80x9d, respectively, without the opposite logic level being optimized.
Usually, it is preferred to regulate to logic xe2x80x9c0xe2x80x9d, and thus according to the algorithm xcex94Vampxcx9cFLxc2x7k1. As appears from the above-mentioned algorithm, the regulation is proportional to k and F, which likewise means that a special case may be that xcx9c is replaced by =.
The driver signal, in response to the pilot tone signal fed to the modulation signal, is regulated to the logic xe2x80x9c0xe2x80x9d or logic xe2x80x9c1xe2x80x9d of the modulation signal in accordance with either
xcex94Vampxcx9cFLxc2x7k1|AL|
or
xcex94ampxcx9cFHxc2x7k2|AH|
where |AL| or |AH| is the numeric amplitude of the pilot tone components detected in the feedback circuit at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, it is additionally made possible to weight the regulation algorithm according to the actual distance from the modulation point to the desired optimum modulation point corresponding to either logic xe2x80x9c1xe2x80x9d or logic xe2x80x9c0xe2x80x9d. This may be an advantage e.g. when starting the system at modulation amplitude zero, as the algorithm tracks the desired modulation point more rapidly.
The photoelectrical converter circuit is connected to and generates a feedback signal for a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted, in response to the pilot tone signal fed from the pilot tone generator and the feedback signal fed from the photoelectrical converter circuit, to generate bias control signals for the bias circuit and driver control signals for the driver circuit to regulate the bias signal and driver signal fed to the Mach-Zehnder modulator, it is possible to perform a combined regulation of a modulator circuit by means of both regulation of the system bias (or DC operating point) and the modulation amplitude.
It is noted that it is not decisive according to the invention how the modulation signals, formed by the driver signal, the bias signal and the pilot tone, are fed to the Mach-Zehnder modulator. The bias signal may e.g. be mixed electrically with the driver signal before the driver signal is fed to the electrodes of the Mach-Zehnder modulator, and the bias signal, the driver signal and optionally the pilot tone may be fed to separate electrodes or sets of electrodes in the Mach-Zehnder modulator, if desired.
The pilot tone signal is fed with the same amplitude to the logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d of the modulation signal, but with opposite phase, it is ensured that the complete system may be dimensioned very uniformly.
However, it should be noted that according to the teaching of the invention, in contrast to known regulation techniques, it is not decisive whether the two pilot tones used have the same phase, frequency, curve shape or amplitude, which may be extremely advantageous in several connections, since it is just necessary to have a pilot tone reference with which the pilot tone component or components fed back are compared.
The feedback circuit detects the pilot tone content of the feedback signal for the logic xe2x80x9c1xe2x80x9d level and the logic xe2x80x9c0xe2x80x9d level, respectively, and determines the phase between said pilot tone content and the pilot tone or tones generated by the pilot tone generator for both the logic xe2x80x9c1xe2x80x9d level and the logic xe2x80x9c0xe2x80x9d level, it is possible to obtain a simple and optimum modulation of the transmission characteristic of the Mach-Zehnder modulator with simultaneous regulation of modulation amplitude and bias. Detection at both the logic xe2x80x9c0xe2x80x9d level and the logic xe2x80x9c1xe2x80x9d level thus ensures continuous retention or regulation in accordance with the ideal or almost ideal modulation levels, said phase additionally giving clear information as to whether modulation is performed on increasing or decreasing half-periods of the transmission characteristic.
The feedback circuit generates a bias control signal for the bias circuit and a driver control signal for the driver circuit to achieve a state of equilibrium in which the phase between said pilot tone content and the pilot tone or tones generated by the pilot tone generator for both the logic xe2x80x9c1xe2x80x9d level and the logic xe2x80x9c0xe2x80x9d level is the same, a simple regulation application is achieved, it being possible beforehand to specify in the regulation algorithm whether phase-locked modulation on the increasing or decreasing half-periods of the transmission characteristic is desired.
It will be appreciated that said regulation algorithm may be supplemented with additional regulation parameters or strategies, it being possible e.g. to introduce a compensation particularly for the bias drift over time. Thus, according to certain guidelines, it may thus be decided to reduce the bias voltage by jumping one period or half a period down the V axis of the transmission characteristic.
Further, situations may occur in which it would be advantageous to change the sign of the bias voltage according to e.g. a mean value consideration of modulation data over a period of time.
The feedback circuit generates a bias control signal for the bias circuit and a driver control signal for the driver circuit to maintain a phase between said pilot tone content and the pilot tone or tones generated by the pilot tone generator for both the logic xe2x80x9c1xe2x80x9d level and logic xe2x80x9c0xe2x80x9d level with the same sign, a very simple and clear application is achieved, as the feedback signal as logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d is specifically used for regulating the driver circuit or the bias circuit, respectively, or vice versa.
Thus, it is noted that the ratio of the phase of the pilot tone component from the pilot tone generator to the phase of the corresponding, detected pilot tone component may have an opposite sign, if this applies to the pilot tone components at logic high as well as logic low. This means that the bias signal puts a DC operating point on the decreasing part of the transmission characteristic.
The decisive point according to the invention is thus to ensure that at logic high and at logic low, respectively, the pilot tone component has the same sign with respect to the added pilot tone, depending on whether the regulation is performed and a function of logic low or logic high. Thus, the regulation of the transient exclusively depends on the selected regulation parameter or parameters.
The bias control signal is provided with a rapid time constant and the drive control signal with a slow time constant, or vice versa, a practical regulation algorithm is achieved, capable of hooking on to an optimum or stable state in a predictable manner.
The bias control signal and the driver control signal are provided sequentially, an alternative embodiment of the invention is achieved, likewise capable of reproducibly hooking on to an optimum and stable state.
The amplitude of the pilot tone content in the feedback signal for the logic high level and the logic low level as well as the associated phase ratio of the corresponding pilot tones regulates the bias circuit and the driver circuit or the driver circuit and the bias circuit, respectively, in accordance with the regulation algorithms
xcex94Vbiasxcx9cFLk1xc2x7|AL|
xcex94ampFHk2xc2x7|AH|
where xcex94Vbias is the change in the bias voltage provided by the regulation, xcex94Vamp is the change in the driver signal provided by the regulation k1 and k2 are a suitably selected constant for the algorithm, and FL and FH assume the value +1 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, is in in-phase with the corresponding pilot tone generated by the pilot tone generator, and FL and FH assume the value xe2x88x921 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and the logic xe2x80x9c1xe2x80x9d, respectively, is in antiphase with the corresponding pilot tone generated by the pilot tone generator, and |AL| and |AH| are the numeric amplitudes of the pilot tone components detected in the feedback circuit at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, a compact and efficient regulation of the modulator circuit is achieved.
It is noted that a special case in accordance with the regulation algorithm is that |AL| and |AH| are set at 1, which means in practice that the amplitudes are not included in the algorithms.
A further special case is that xe2x80x9cxcx9cxe2x80x9d is replaced by xe2x80x9c=xe2x80x9d which makes the algorithm extremely simple and efficient.
All the last-mentioned algorithms are surprisingly simple, as the determined reference point of logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, individually controls bias and amplitude, respectively.
The amplitude of the pilot tone content of the feedback signal for the logic high level and the logic low level as well as the associated phase ratio of the corresponding pilot tones regulates the bias circuit and the driver circuit or the driver circuit and the bias circuit, respectively, in accordance with the regulation algorithms
xcex94VAMPxcx9cFLxc2x7k1xc2x7|AL|
xcex94biasxcx9cFHk2xc2x7|AH|
where xcex94Vbias is the change in the bias voltage provided by the regulation xcex94Vamp is the change in the driver signal provided by the regulation, k1 and k2 are a suitably selected constant for the algorithm, FL and FH assume the value +1 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, is in-phase with the corresponding pilot tone generated by the pilot tone generator, and FL and FH assume the value xe2x88x921 when the pilot tone component at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively are in antiphase with the corresponding pilot tone generated by the pilot tone generator, and |AL| and |AH| are the numeric amplitudes of the pilot tone components detected in the feedback circuit at logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, respectively, a corresponding simple algorithm is achieved, as, in this case, the amplitude regulation is locked directly to logic xe2x80x9c0xe2x80x9d and the bias is locked directly to logic xe2x80x9c1xe2x80x9d.
The photoelectrical converter circuit is connected to a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted to generate driver control signals for the driver circuit connected therewith in response to detected pilot tone components in the feedback signal, an implementation of the above-mentioned method is achieved for a modulator circuit with amplitude regulation.
The photoelectrical converter circuit is connected to a feedback circuit which is also connected to the pilot tone generator, said feedback circuit being adapted to generate bias control signals for the bias circuit connected therewith and driver control signals for the driver circuit connected therewith in response to detected pilot tone components in the feedback signal, an implementation of the above-mentioned method is achieved for a modulator circuit with combined bias and amplitude regulation.